The present invention relates to an improved control system for hydraulic lifts.
Hydraulic bucket lifts are generally comprised of a ground station or base, which may be mounted on a bed of a truck where the lift is to be transportable, and a bucket for holding one of more persons. Two or more pivotally interconnected mechanical arms extend between the base and the bucket, and are movable by hydraulic cylinders to raise and to lower the bucket, and the base is rotatable by one or more hydraulic cylinders. Controls, which are normally located both at the ground station and in the bucket, operate fluid flow valves connected with the hydraulic cylinders, whereby the cylinders may be selectively operated to move the bucket to desired positions.
Conventionally, hydraulic control systems for the cylinders are of the open center type. With such systems, the fluid constantly flows in a series path through the valves, and to operate a cylinder the fluid flow at the valve therefor is blocked. This is accomplished by directing the fluid flow at each valve through associated controls at the ground station and in the bucket, with actuation at one of the controls blocking the fluid flow at the valve. As a consequence of the hydraulic fluid constantly flowing through the system at relatively high back pressure on the order of 600 psi, heat is generated within the system and shortens the life the components thereof, and a pump for the fluid is expensive to operate and wears out rapidly. Also, only one hydraulic cylinder at a time may ordinarily be operated, since during its operation fluid flow to the remaining "downstream" portion of the system is blocked. Alternatively, a second cylinder may be simultaneously operated with the fluid exhausted from the first, but then both cylinders operate at a greatly reduced speed.
As fluid lines to the controls carry the full fluid flow and pressures of the system, they are of relatively large diameter and expensive. The lines to controls in the bucket extend in a tight bundle along the mechanical arms, and are particularly subject to flexing and wear at the pivot points of the arms. Replacement of these lines is time consuming and difficult. More importantly, because the lines carry full system fluid pressures, bursting of a line in proximity with a control could cause injury to a nearby operator.